Ultrafast underwater self-healing piezo-ionic elastomer via dynamic hydrophobic-hydrolytic domains

The development of advanced materials capable of autonomous self-healing and mechanical stimulus sensing in aquatic environments holds great promise for applications in underwater soft electronics, underwater robotics, and water-resistant human-machine interfaces. However, achieving superior autonomous self-healing properties and effective sensing simultaneously in an aquatic environment is rarely feasible. Here, we present an ultrafast underwater molecularly engineered self-healing piezo-ionic elastomer inspired by the cephalopod’s suckers, which possess self-healing properties and mechanosensitive ion channels. Through strategic engineering of hydrophobic C–F groups, hydrolytic boronate ester bonds, and ions, the material achieves outstanding self-healing efficiencies, with speeds of 94.5% (9.1 µm/min) in air and 89.6% (13.3 µm/min) underwater, coupled with remarkable pressure sensitivity (18.1 kPa–1) for sensing performance. Furthermore, integration of this mechanosensitive device into an underwater submarine for signal transmission and light emitting diode modulation demonstrates its potential for underwater robotics and smarter human-machine interactions.

1.The authors adjusted the content of C-F groups to achieve the best synergy for self-healing.However, the changing trends in air and underwater are quite different in Figure 2c.This difference should be more clearly explained.
2. Introducing dynamic bonds like B-O for room-temperature healing generally sacrifice the elastic properties, which are important for durable sensing applications.The reversible compressive experiments on different samples should be provided to study its effect on elasticity.
3. How about the self-healing efficiency at the totally dried condition?This can exclude the effect of water, which help highlight the role of hydrolytic reaction underwater.
4. The vertical labels for Figure 2c and d should be unified for better comparison.
5. The fitted IR peak areas do not directly represent the actual contents, due to the difference of absorption coefficient at different wavenumbers.Corresponding revision should be made in Figure 2b (the authors may refer to: Adv.Mater.2023, 35, 2209581).Also in Figure 3, Intensity should be "Absorbance" for IR spectra.
6.The formats for all the references should be carefully checked to comply with the journal's requirements.
Reviewer #3 (Remarks to the Author): The authors synthesized an ultrafast underwater molecularly engineered self-healing piezo-ionic elastomer (MESHPIE) inspired by the cephalopod's suckers, which possessed self-healing properties and mechanosensitive ion channels.They integrated a material-based device into an underwater submarine for signal transmission and LED illumination to demonstrate its potential for underwater robotics.However, the MESHPIE-based device characterizations and demonstrations are insufficient and require further improvements and analysis.Major revision is needed before further consideration.Below are some comments: 1.It is mentioned in the introduction that "However, these materials exhibited relatively low self-healing efficiencies and speeds owing to the limited synergistic effect between the hydrophobic groups and dynamic bonds".Corresponding data need to be provided for the relatively low self-healing efficiencies and speeds here.
2. The labels in Figure 1 need to be improved.For example, the cations in Figure 1c are Ca2+ ions, which need to be labeled in the figure .3. In Figure 2f, quantitative characterizations should be provided to demonstrate the absence of water leakage when the material is loaded.2i compares the self-healing speeds of different self-healing materials at different temperatures.The author tested the self-healing speed at room temperature.How about the effect of temperature on its self-healing speed?And how about the self-healing speed at higher temperature? 5.In Figure 4b, the authors state that the ion trap and release before and after inducing pressure generate a low initial capacitance and a high final capacitance, respectively, owing to the efficient pumping of ions.Is there evidence that the material undergoes ion release under pressure stimulation?6.In Figure 4h, the response time of the device is 281 ms, which is slower than other ionic conductorsbased devices.Why is the response time so slow?Is it possible to improve the response time? 7.Although the MESHPIE can be placed in water, other components of the MESHPIE-based devices might fail?Please comment on this.

Figure
8. If the devices based on this material are to be used in water or air, their stability is very important.How about the stability of the material in water?How about the shelf-life?9.The demonstration in Figure 5e does not effectively reflect the advantages of the MESHPIE-based device.Most ionic conductors-based pressure sensor can do.

Self − healing time(min)
The self-healing efficiency can be defined as the ability of a material to recover a particular property after damage relative to the original or the undamaged material (Chem.Soc.Rev. 42, 7446-7467 (2013)).This quantifies the evaluation of a healed material to determine the extent of recovery of initial properties after mechanical damage.The self-healing efficiency in this work was calculated from Fig. 2g and supplementary Fig. 13, based on the ratio of (the toughness of healed sample after) / (the toughness of original sample) x 100%.The formula is as shown below: Self − healing efficiency = Toughness ℎ Toughness Original × 100% To address the reviewer's concern with regard to error bars, we have replaced Figs.2c and 2d with Figs.
R1a and R1b respectively in the revised manuscript.To clarify the basis of the self-healing evaluation methods, we have added the following sentences and the equations for both self-healing speed and selfhealing efficiency to the revised Supplementary Information on page 3 and the following references to the Supplementary References.
"The self-healing speed (µm/min) can be defined as the rate at which the damaged sample achieves maximum self-healing efficiency with or without external stimuli.The self-healing speed is often determined from the ratio of [notch size] (µm) / self-healing time (min) 1,2 .However, in this study, the sample was cut entirely with the knife penetrating through to the lower part, hence, the notch size was determined by the thickness of the sample.Furthermore, the self-healing efficiency was calculated from the ratio of (the toughness of healed sample / the toughness of original sample) 3 x 100%.The formulae are as follows:" Self − healing speed = Thickness(µm) Self−healing time(min) (1) [1] Ying, W.

Q4)
At the end of the paper, the underwater application test of the sensor is given.It is recommended to supplement the electrical test of the above materials with underwater electrical tests.

Response:
We appreciate the reviewer for this valuable comment.In our study the impedance and capacitance are great experiments that characterize the electrical performance of our material.In order to support the electrical performance of our material in underwater conditions, we have conducted impedance (Nyquist plots) and capacitance tests (Fig. R3a and R3b, respectively) of MESHPIE in both ambient and underwater conditions.The results showcase the exceptional underwater electrical performance and stability of MESHPIE, ensuring the stable underwater application of MESHPIE-based devices.
Additionally, Supplementary Movie 6 (see detail explanation in Supplementary Note 6) not only supports the electrical self-healing of the MESHPIE-based device, but also affirms the electrical performance of the device in underwater conditions.
To supplement the underwater electrical tests, we have added Fig. R3a and R3b as Supplementary Fig. 22, and the following sentences on page 8 of the revised Supplementary Information.
"to support the underwater electrical performance of MESHPIE, capacitance (Supplementary Fig. 22a) and impedance (Supplementary Fig. 22b) tests were conducted in both ambient and underwater conditions.Response: We thank the reviewer for the valuable comments.Actually, the hydrophobic structure permeates the entire polyurethane material owing to the introduction of C-F side chain group in soft segment, as depicted in Fig. 1g, original manuscript.Thus, the entire MESHPIE exhibits hydrophobic 6 properties, as evidenced by the water contact angles depicted in Figs.2c-d, original manuscript, and Supplementary Fig. 6.Please refer to this reference (Mater.Horiz., 8, 2761-2770 (2021)) for example of this phenomenon.

The results showcase the exceptional underwater electrical performance and stability of MESHPIE, ensuring the stable underwater application of MESHPIE-based devices."
Nevertheless, in this study, our discussion focuses on the interaction between MESHPIE and water molecules action on its surface.Fig. 1d and Fig. 2a highlight the hydrophobic effect acting as a barrier to repel the majority of water molecule ingress.As discussed in the original manuscript, increasing the C-F group density in MESHPIE increases the contact angle, indicating excellent hydrophobicity.As proof of hydrophobicity, Supplementary Fig. 6 is included here as Fig. R4 for easy reference to the reviewer.

Reply to Reviewer #2
This work introduced an ultrafast underwater self-healing piezo-ionic elastomer via the synergy of hydrophobicity and B-O hydrolytic reaction.The optimized piezo-ionic elastomer showed impressive selfhealing efficiencies and sensing performance.The manuscript was also well organized with dense data for supporting their conclusions.I overall recommend the publication of this work in Nature Communications.However, before acceptance, a major revision is necessary for further polishing their findings.
Response: We appreciate the reviewer for the constructive comments and recommendation of our manuscript.We have revised the manuscript according to the reviewer's comments.

Q1)
The authors adjusted the content of C-F groups to achieve the best synergy for self-healing.However, the changing trends in air and underwater are quite different in Figure 2c.This difference should be more clearly explained.

Response:
We thank the reviewer for asking us to clarify the explanation on Fig. 2c.As discussed in the original manuscript, the introduction of higher C-F groups not only leads to the formation of dense hydrophobicity, but also enhances dipole-dipole interactions.The changing trend of self-healing in air is attributed to enhanced dipole-dipole interactions with the introduction of higher C-F groups in the MESHEs as depicted in Fig. 2c.On the other hand, the underwater self-healing speed of the MESHEs depends largely on the reversible hydrolysis/re-esterification reactions of boronate ester bond caused by the ingress of small quantity of water molecules.Thus, the changing trend of underwater self-healing speed increases with higher C-F group content, however, MESHE3, with the highest C-F group content exhibited a drastic decrease in underwater self-healing speed, owing to denser hydrophobicity which limited the amount of water ingress necessary for effective self-healing through boronate ester bond hydrolysis.

Q2) Introducing dynamic bonds like B-O for room-temperature healing generally sacrifice the elastic properties, which are important for durable sensing applications. The reversible compressive experiments
on different samples should be provided to study its effect on elasticity.

Response:
We appreciate the reviewer for this important comment.In our system, the chain extender GBDB containing B-O bonds, in conjunction with IPDI, forms the hard segment of polyurethane, providing physical cross-linking points in the polyurethane network to ensure excellent mechanical performance.In Fig. R5, BPU is a polyurethane composed of monomers IPDI, HTPB, and GBDB.The monomer ratios of BPUs are presented in Table R1, showing that BPUs have the same soft segment content and a gradually increasing chain extender weight, indicating an augmented dynamic bond content.
Therefore, in our system, the mechanical modulus of polyurethane increases with the dynamic B-O bond content, rather than being compromised, as depicted in Fig. R5 and Table R2.Furthermore, to address the reviewer's concern, we conducted compression experiments on MESHPIE to investigate its elasticity, as shown in Fig. R6.The MESHPIE exhibited excellent elastic recovery without obvious plastic deformation.The nearly similar compressive stress-strain loading/unloading curves emphasize the stability and outstanding fatigue resistance of MESHPIE, an indicative of its remarkable elastic recovery capabilities (Fig. R6a).Additionally, we subjected the material to 100 consecutive compression cycles to further evaluate its fatigue resistance (Fig. R6b).Following 100 cycles of compression tests, the stress attenuation registered a mere 1.93%, attesting to the material's outstanding fatigue resistance.
To support the elastic behavior of our material, we have included the following sentences on page 4 and Fig. R6 as Supplementary Fig. 13 in the revised Supplementary Information.
"To examine the reversible compressibility of MESHPIE for effective device performance, compressive stress-strain tests were performed with cyclic loading/unloading strain (Supplementary Fig. 13).The compressive test exhibited similar loading/unloading curves emphasizing the stability and outstanding fatigue resistance of MESHPIE, indicative of its remarkable elastic recovery capabilities (Supplementary Fig. 13a).Additionally, we subjected the material to 100 consecutive compression cycles to further evaluate its fatigue resistance (Supplementary Fig. 13b).Following 100 cycles of compression tests, the stress attenuation registered a mere 1.93%, attesting to the material's outstanding fatigue resistance."peak at 1360 cm -1 represents the boron oxygen bond (B-O) of boronate esters before immersing in water.
After immersion in deionized water for 30 minutes or more, a new characteristic peak attributed to boronic acid groups (-B(OH)2) appeared at 1341 cm -1 , indicating hydrolysis of the boronate ester bonds.Therefore, the fractional area of the boronic acid group peaks, obtained through deconvolution calculations of the characteristic peaks at different immersion times, reflects the degree of hydrolysis at different immersion times.

Q6)
The formats for all the references should be carefully checked to comply with the journal's requirements.
Response: We appreciate the reviewer for pointing this out.We have revised the format of the references to comply with Nature Communications' requirements.

Q3)
In Figure 2f, quantitative characterizations should be provided to demonstrate the absence of water leakage when the material is loaded.

Response:
We thank the reviewer for this comment.The experiment in Fig. 2f original Manuscript, was conducted to demonstrate that not only is our self-healing material capable of autonomous self-healing of scratch and microscale damages but also can self-heal macroscale damages (Nat Rev Mater 5, 562-583 (2020); European Polymer Journal 53, 118-125 (2014)) in underwater conditions.As can be vividly seen in the original Supplementary Movie 1, no breaking or water leakage was observed even under pressure.Nonetheless, to clarify the absence of water leakage, we have reproduced this demonstration over an electronic scale to quantitatively characterize the absence of water leakage even under applied pressure.
As clearly shown in Fig. R13 and revised Supplementary Movie 1, no water drop was recorded by the electronic scale when pressure was applied, as evidenced by the stable readings.This demonstrates and confirms the absence of water leakage owing to the excellent underwater self-healing capabilities of our material.
To address the reviewer's concern, we have replaced Fig. 2 with Fig. R13 in the revised Manuscript and Supplementary Movie 1 with the revised Supplementary Movie 1. Q4) Figure 2i compares the self-healing speeds of different self-healing materials at different temperatures.The author tested the self-healing speed at room temperature.How about the effect of temperature on its self-healing speed?And how about the self-healing speed at higher temperature?
Response: We appreciate the reviewer for raising this important point.It is well known that elevated temperatures cause the generation of high segmental chain mobility to facilitate faster healing of the damage area (Nat Rev Mater 5, 562-583 (2020)).As expected, at higher temperatures, our material exhibited faster healing speeds owing to enhanced segmental chain mobility (Fig. R14).

Q5)
In Figure 4b, the authors state that the ion trap and release before and after inducing pressure generate a low initial capacitance and a high final capacitance, respectively, owing to the efficient pumping of ions.Is there evidence that the material undergoes ion release under pressure stimulation?
Response: We thank the reviewer for the valuable comments.As discussed in the original manuscript under the section "Characterization of piezo-ionic dynamics" and Supplementary Note 4, the relationship between piezo-ionic dynamics and their corresponding changes in complex impedance behavior were analyzed to provide clear evidence to support the ion release under pressure stimulation.Please see these sections for detailed explanations.Nonetheless, the decrease in impedance with increasing pressure (Supplementary Fig. 16a, original Supplementary Information), owing to release of more free ions that were initially trapped, and the shift of charge relaxation frequency (τ -1 ) in the Bode plot towards higher frequencies (Fig. 4c, original Manuscript) owing to faster ionic atmosphere relaxation indicating increased free ion concentration owing to ion release under pressure in MESHPIE, which was not an observed tendency in NFPU-IL (without C-F groups)-based device (Supplementary Fig. 16b and 16c, original Supplementary Information), are clear evidence to support that the material undergoes ion release under pressure stimulation.

Q6)
In Figure 4h, the response time of the device is 281 ms, which is slower than other ionic conductorsbased devices.Why is the response time so slow?Is it possible to improve the response time?
Response: We appreciate the comment from the reviewer.The slow response time recorded in Fig. 4h original Manuscript, can be attributed to the lower frequency (100 Hz) utilized to achieve the most efficient electric double layer (EDL) formation (Science 370, 961-965 (2020)), which resulted in slower response time due to the gradual changes in capacitance at lower frequency.Increasing the frequency to 1000 Hz results in a faster response time of 78 ms (Fig. R15).
To address the reviewer's concern, we have replaced Fig. 4h with Fig. R15 in the revised manuscript.Response: We thank the reviewer for this comment.As discussed on page 9 of the original manuscript, the MESHPIE-based device consists of MESHPIE sandwiched between two deformable Ag nanowire (AgNW)/MESHE2 electrodes.The electrodes were developed using superb hydrophobic MESHE2 (the base material for developing MESHPIE) as substrate to protect the AgNW from direct interference with water molecules (see "Method" section subheading "Fabrication of self-healing electrodes" for details).
Moreover, to establish connection with measuring instruments, encapsulated Ag wires were used to prevent any interference against water molecules.Therefore, all components of the MESHPIE-based device were hydrophobically insulated from water molecules, thus no failure was detected.

Q8)
If the devices based on this material are to be used in water or air, their stability is very important.How about the stability of the material in water?How about the shelf-life?Response: We thank the reviewer for this comment.We totally agree that some ionic conductor-based pressure sensors can perform this demonstration.Nevertheless, we performed this demonstration in this study to affirm that our material is also touch-sensitive showcasing its capability to detect human interactive activities through touch, owing to its remarkable mechanosensitivity.

Figure. R1 | a
Figure.R1 | a Self-healing speeds of the various MESHEs in both ambient and underwater conditions.b Selfhealing speeds of the various MESHPIEs with different IL concentrations in both ambient and underwater conditions.

Fig. R2 |
Fig. R2 | Demonstration of practical device performance in ambient and aquatic environments.a Changes in LED intensities under touch-induced pressures, before and after self-healing situations.b Illustration of MESHPIEbased device architecture, which is incorporated into an underwater toy submarine for practical demonstration.c Sensing responses upon impact with underwater object (wall of the container).d Photographs showing the visualization of changes in LED intensity upon impact with underwater object.e Pressure response of self-healed device after encountering severe damage underwater.

Fig. R3 | a
Fig. R3 | a Capacitance of MESHPIE in ambient and underwater conditions (applied bias voltage of 1 V at 20 Hz, no applied pressure).b Impedance Nyquist plots of MESHPIE in ambient and underwater conditions.

Fig. R4 |
Fig. R4 | Changes of water contact angles.Images of the water contact angles of the various PUs and calculated angles representing the changes in their hydrophobicity after being immersed in water for five consecutive days.

Fig. R6 | a
Fig. R6 | a Sequential cyclic compressive stress-strain curves of MESHPIE at 25% strain.b Fatigue resistance test of MESHPIE by sequential cyclic compression test (100 times).
, Fig. R10, and Fig. R11, respectively, in the revised Manuscript and revised Supplementary Information.

Fig. R8 |
Fig. R8 | Analysis of the hydrolytic behavior of the various MESHEs at different immersion times.This graph represents the calculated peak area fraction of boronic acid group as a function of the soaking times.

Fig. R10 |
Fig. R10 | ATR-FTIR spectra of a, MESHE1, b, MESHE2 and c, MESHE3 in the spectral regions of 1380-1335 cm −1 attributed to B-O group and B(OH) 2 group at different water immersion times.

Fig. R12 |
Fig. R12 | Molecular chemistry of the cephalopod and conceptual design principle of MESHPIE.a Schematics of cephalopod and ring teeth (RT) structures.b The RT proteins within the cephalopod's suckers consist of numerous hydrogen bonds and hydrophobic core, which enable self-healing in both ambient and aquatic conditions The hydrogen bonds self-assemble into segmented semicrystalline morphology (amorphous and β-sheets).c Unique mechanoreceptor NompC composed of tethered ion channels observed in cephalopod's suckers.d Chemical structure representation of MESHPIE consisting of hydrophobic domain and reversible boronate ester bonds for self-healing in both ambient and aquatic environments.e Schematic illustration depicting the structure of MESHPIE, consisting of hard and soft segments, emulating the RT protein structure of the cephalopod.f Piezo-ionic

Fig. R13 |
Fig. R13 | Cut and spliced scenario of two different MESHPIE pieces, healed together underwater to withstand breaking under pressure.

Fig. R16 |
Fig. R16 | Changes of water contact angles.Images of the water contact angles of the various PUs and calculated angles representing the changes in their hydrophobicity after being immersed in water for five consecutive days.

Fig. R17 |
Fig. R17 | Mass change rate.Changes in the mass of the MESHPIE-based device after being immersed in water for different days.

Fig. R18 |
Fig. R18 | Changes in impedance (electrical properties) after soaking in water.Impedance Nyquist plots of MESHPIE-based device after being immersed in water for different days.

31, 2009869
Bin et al.A Biologically Muscle-Inspired Polyurethane with Super-Tough, Thermal Reparable and Self-Healing Capabilities for Stretchable Electronics.Adv.Funct.Mater.

Table R1 .
The componential mole ratios of various BPUs.

Table R2 .
The Mechanical properties of various BPUs.